The present invention relates to rotary fluid pressure devices, and more particularly, to such devices of the type including a fluid displacement mechanism which comprises a gerotor gear set.
Although the present invention may be included in a gerotor type device being utilized as a pump, it is especially adapted for use in a low-speed, high-torque gerotor motor, and will be described in connection therewith.
For years, many of the gerotor motors made and sold commercially, both by the assignee of the present invention as well as by others, have had the motor valving disposed "forwardly" of the gerotor gear set (i.e., toward the output shaft end of the motor), thus having nothing disposed "rearwardly" of the gerotor gear set except for an end cap. The present invention is especially adapted for use with gerotor motors of this type, and will be illustrated and described in connection therewith.
In many vehicle applications for low-speed, high-torque gerotor motors, it is desirable for the motor to have some sort of parking brake or parking lock, the term "lock" being preferred because it is intended that the parking lock be engaged only after the vehicle is stopped. In other words, such parking lock devices are not intended to be dynamic brakes, which would be engaged while the vehicle is moving, to bring the vehicle to a stop.
For many years, those skilled in the art have attempted to incorporate brake and lock devices into gerotor motors. Examples of such devices are illustrated and described in U.S. Pat. Nos. 3,616,882 and 4,981,423. In the device of U.S. Pat. No. 3,616,882, a braking element is disposed adjacent the forward end of the gerotor star, and is biased by fluid pressure into frictional engagement therewith. Such an arrangement involves a certain degree of unpredictability of performance, in view of variations in clearances, etc. Such an arrangement also requires a substantial redesign of the wear plate and forward bearing housing of the motor. In the device of U.S. Pat. No. 4,981,423, there is a multi-disc brake assembly which is of the "spring-applied, pressure-released" type. The arrangement of U.S. Pat. No. 4,981,423 also requires almost total redesign of the forward bearing housing, and also results in a much larger bearing housing. In addition, the disc pack is in splined engagement with the output shaft and, therefore, must be able to brake or hold the full output torque of the motor, thus necessitating that the discs, the spring, and the apply/release piston all be relatively larger.
A different approach to a parking lock was illustrated and described in U.S. Pat. No. 4,597,476, assigned to the assignee of the present invention and incorporated herein by reference. In the incorporated patent, the lock mechanism included a roller which could be moved from its normal, retracted position, which would permit normal orbital motion of the gerotor star, to a locked position, disposed at least partially within one of the volume chambers of the gerotor gear set, thus preventing normal orbital motion of the star. Although the device of the incorporated patent provides a generally satisfactory locking function, having such a member engage the star profile is generally considered undesirable, and the mechanism requires a substantial redesign of the gerotor ring, in which the lock actuator is located.
Also known to the assignee of the present invention, although perhaps not known publicly, is a parking lock arrangement in which a spring-applied, pressure-released piston is disposed in the end cap, and includes a cylindrical lock member which is received within a cylindrical bore in the end of the orbiting and rotating dogbone shaft. Although the arrangement described is generally functional, it has two significant disadvantages. The first is that, because the lock member has to fit within a bore defined by the dogbone shaft, it is necessarily fairly small in diameter, and has somewhat limited torque capacity. The second is that the cylindrical lock member within the cylindrical bore makes it nearly impossible to disengage the lock member under load.